1. Field of Invention
The present invention relates generally to the field of seismic data acquisition, and particularly to cables adapted to lie on a seabed and methods of stabilizing them when exposed to currents.
2. Related Art
Marine seismic exploration investigates and maps the structure and character of subsurface geological formations underlying a body of water. In so-called seabed seismic, a cable containing seismic receivers is deployed onto the seabed from a surface vessel. Older ocean bottom cables were comprised only of a hydrophone and a vertically oriented geophone strapped to the outside. More recent versions of seismic seabed cables typically comprise an instrumented cable packed with receivers, similar to the streamers that are towed in conventional marine surveys, but designed to operate on the seafloor. One seabed cable, known under the trade designation “NESSIE 4C”, available from WesternGeco LLC, Houston, Tex., contains multiple sensing units each containing one hydrophone and three orthogonally oriented geophones inside the cable, distributing their weight for optimal coupling to the seafloor. This design was an improvement over the older ocean bottom cables. Other seabed seismic cable designs comprise separate support and signal cables, which help to acoustically decouple the sensors from the support cable. The function of support cable and seismic cable are sometime included in a single construction during the manufacture of the cable, and signal leads in this type of cable may be exposed and connected to sensors units.
No matter what design of the seismic seabed cable, they are subject to instability in the presence of strong water currents at or just above the seafloor. Tidal currents, river currents, strong underwater streams, and bad weather (waves) are but a few examples. Water currents induce vibrations that may be transmitted to the sensors. The noise level on the sensors may increase, which may degrade data quality and may actually stop data acquisition. Furthermore, the coupling of the sensor housing to the seafloor may become poor and inconsistent, which may degrade data quality. Also, any significant cable movement may require repositioning of the cable, losing time and may require picking up and re-deploying the cable. Prior solutions have focused on increasing the weight of the cables, but there are serious drawbacks to that approach. Good coupling of sensor housings to the seabed is optimized by a larger weight contrast between the cable and the housings so that there is minimal cable interference. However, heavily weighted cables may translate to higher tension during deployment and retrieval, unless the deployment or retrieval vessel speed is closely matched to the cable deployment rate, limiting efficiency and requiring much stricter mechanical specifications for the cables.
Fairing is a well-known technique to reduce drag of cables being pulled or deployed through water and to avoid “strumming”, a term of art used to describe the action of currents on a cable. Fairing may provide other advantages: it gives mechanical protection to the cable against cuts, wear and abrasion. Fairing has not been used to increase stability of a seismic cable laying at the seafloor. Known seabed seismic cables have no provision for fluid to pas between the cable and the seabed. Sensor housings are known that are attached to the cable, and some have exterior cleats or other members promoting adherence to the seabed, but the cable itself is still able to move about and rotate. From the above it is evident that there is a need in the art for improvement in seismic seabed cables, and in reducing the effects of strong currents thereon.